For fill level measurement of a liquid or a bulk good in a container, so-called oscillatory forks are frequently used. In such case, there exist a number of different forms of embodiment, which differ mainly in the shape and length of the oscillatory fork. Such oscillatory forks are composed of two rods and are placed on a membrane, which is excited to oscillations by a drive unit. The two rods are thereby excited to opposite phase oscillations. The excitation is done with the resonance frequency of the oscillatory system. The oscillation is weakened when the oscillatory fork comes in contact with the measured medium. Due to the extra mass, which must be moved when the oscillating fork reaches into a liquid medium, the oscillation frequency decreases. In the case of media with lower density, such as, for example, gases, this effect is negligible. In the case of all viscous liquid media, a damping of the oscillation amplitude occurs as a result of friction. The amplitude change and/or frequency change is evaluated as a function of the medium. Additionally, the phase shift between the drive voltage and received voltage from the oscillatory rods moved to oscillations by the driving is evaluated, since this is likewise changed in the medium.
In containers, two such oscillatory forks are often applied, with one being placed in the lower region of the container and one in the upper region. In the normal state, the oscillatory fork mounted above oscillates in air, and provides an overflow protection, while the oscillatory fork mounted below oscillates in normal operation in the medium, and provides protection against running dry. Such oscillatory forks function excellently in liquids which do not form accretions and in bulk goods with small particle sizes relative to the oscillatory fork. In the case of media which form accretions on the oscillatory fork, after a certain accretion thickness, bridges form, which reach from the one rod to the other. These influence the oscillatory system and ultimately lead to the oscillatory rods no longer oscillating, and thus to measuring being no longer possible.
Oscillatory forks can also be applied for determining the density and/or the viscosity of a liquid medium. In order to be able unequivocally to determine a measured variable, it is, however, necessary to hold the boundary conditions constant and, respectively, to determine disturbance variables. An example of such boundary conditions or disturbing variables in the case of determining density and viscosity is fill level. In the determining of viscosity, a variable density of the medium, for example for reasons of temperature fluctuations, represents a disturbing variable. In order nevertheless to be able to determine viscosity, density is held constant, density is determined with a separate measuring device, or a measuring method is selected, which compensates for density dependencies. The latter is often connected with a large electronic effort. Often, it is not possible to hold the boundary conditions constant, so that at least a second measuring device is necessary for determining the disturbance variables. This requires at least one additional process connection, which not only brings with it extra costs, but also introduces hygienic risks, depending on the field of application.
In the case of bulk goods composed of coarse particles or liquids containing coarse particle, in the case of which the diameter of the particles is about equal to the distance between the two rods of the oscillatory fork, it occurs that particles jam between the rods. This, too, leads to a malfunctioning of the oscillatory fork. As a rule, in both cases, an error report is produced, which, for safety reasons, turns off the process.
A non-vibronic principle for fill level measurement of liquids is that wherein a rod is introduced into the container. The rod protrudes far into the container, and which is provided with a plurality of so-called floats. The floats then arrange themselves at a determined height corresponding to fill level of the liquid.
Due to the application of a plurality of floats on the rod, with this apparatus, not only a fill level can be determined, but also, for example, phase boundaries in the case of multiphase mixtures. A disadvantage in the case of this method is, however that the floats are susceptible to wear.
From the publication DE 3215040 C2, an apparatus for vibronic fill level measurement is known, which is composed of a measuring tube, which is introduced into the container and excited to oscillations by a drive lying in the interior of the measuring tube. The measuring tube is cylindrical, wherein the base is round or elliptical. A jamming of particles does not present a problem in the case of this apparatus. A disadvantage, however, is that, for exciting the measuring tube, relatively high frequencies are necessary, which strain the drive and the electronic components. Additionally, only low oscillation amplitudes are attainable.